Apparatus for and method of optical detection and analysis of an object

ABSTRACT

An apparatus for detection and analysis of optical systems. Prior art systems exploit the phenomenon of photon scattering in order to range the distance of objects from the ranging apparatus. However, no information other than the distance of the target object is obtained from such systems. There is therefore provided an apparatus that exploits the technique of single photon counting and the phenomenon of retro reflection to provide information about a target optical system. Such information can be analysed and compared against known optical systems to provide a means of identification. Alternatively such information can be used as a method of quality control when constructing precision optical instruments such as telescopes or microscopes.

[0001] The present invention relates to an apparatus for opticaldetection and analysis and in particular an apparatus for identificationof a selected optical system.

[0002] Light beams will interact with any object in such a way that someenergy will be diverted or converted. Depending on whether the objectconsists of refractive or reflective interfaces such interactivemechanisms will include scattering, reflection and refraction, all ofwhich differ in efficiency within any given optical system depending onthe light wavelength and the physical characteristics of the object.

[0003] For example, the phenomenon of scattering is what underlies theoperation of known optical range-finding systems such as a LIDAR (LightDistancing and Ranging), which is an optical equivalent of a RADARsystem, or a Laser Range Finder system (LRF). In LIDAR/LRF systems alight pulse is sent out from the system which then waits for any returnsignals. By timing the interval between transmission and reception thedistance to the target object can be calculated. The operation of suchsystems is covered in “Introduction to Radar Systems” by M. I. Skolnik(McGraw Hill).

[0004] A problem with existing LRF/LIDAR systems is that they provide noinformation about a target system other than how far away it is. Targetidentification is crucial in, for example reconnaissance andsurveillance situations and therefore it would be desirable for anoptical range-finding system to also be capable of identifying the typeof optical system being ranged.

[0005] Positional analysis of optical systems is also important in theproduction of complex optical systems, such as microscopes, telescopesetc., from the point of view of quality control. Currently, the positionof optical components within such systems must be inferred from theoptical performance of the system measured via interferometers orModulation Transfer Function (MTF) equipment. It would therefore bedesirable to have a device capable of directly measuring the position ofoptical components within a built up optical system.

[0006] It is possible to exploit one of the other interactive mechanismsmentioned above in order to obtain more information about a targetsystem. If an optical system is illuminated by a light source some lightwill be reflected back towards the light source—this is the phenomenonof RetroReflection. If a light detector is used in conjunction with alight source then the presence of an optical system can be detected.

[0007] The simplest example of a system in which RetroReflection occursis an everyday simple mirror. Another example of a RetroReflectiongenerator is the “Cat's Eye” system used on roads. In this device light,from car headlights, is focused onto the surface of a reflector andretroreflected out again.

[0008] The presence of a “mirrored” reflector is not necessary forRetroReflection to occur. Whenever an optical wavefront encounters achange in refractive index, it changes its velocity slightly since thespeed of light is different in different materials. If the wavefrontencounters a refractive surface at an angle the net result is that thetransmitted beam of light bends, the process of refraction. However,this simple view of the interaction takes no account of the imperfectionof the interface between the two materials. In much the same way thatelectrical cables need to be impedance matched into their terminatingloads then light waves need to be impedance matched across refractiveboundaries. For the case of a light ray which has normal incidence at arefractive boundary it was shown by Fresnel that the refractive indices,n₁ and n₂, of the materials on each side of the boundary cause a certainproportion of the incident light to be reflected in the ratio:r = (n₁ − n₂)²/(n₁ + n₂)²

[0009] For the case of a vacuum/glass transition (n₁=1.000; n₂=1.5) thissurface reflection ratio is 4.2% and there will therefore beRetroReflection generation. In other words if a glass or plastic systemis illuminated by a light source there will still be RetroReflectionwhich can be exploited to obtain information about the target.

[0010] Most optical systems will, however, have some sort of structure.For example, binoculars have an internal structure consisting of aseries of lenses and prisms all of which will RetroReflect. Since eachoptical surface will RetroReflect some of the incident irradiating lightthere will be multiple retro-reflected light signals which will varybetween different optical systems. Therefore, different optical systemswill have different “optical signatures” and it should be possible toanalyse the signature to determine the characteristics of the targetoptical system. It should be noted that the term “optical system” neednot refer to a system that consists of a series of glass lenses—anyobject that has a series of reflecting surfaces, be they “mirrored”surfaces or glass/air transitions or otherwise, should be considered asan optical system. The human eye, for example, will also generate anoptical signature.

[0011] However, the method by which the “optical signature” is extractedfrom the reflected light will be crucial. This is because even in thesimplest optical systems the returning combination of wavefronts islikely to be very complex due to multiple internal reflections. If ahighly coherent light source, such as a laser, is used then thereflected wavefronts will be able to vectorially add and subtractresulting in an interference pattern (this is due to the fact thatspatial coherence means that the reflected wavefronts bear a fixedrelationship to one another and are therefore able to opticallyinterfere). Attempting to use such an optical interference pattern asthe “optical signature” will result in problems due to the shortwavelength of light. Firstly, the components of optical systems do notnormally require to be assembled to interferometrically close tolerancesand so optical systems produced successively on the same production linewould have differing signatures. Secondly, the wavelength of light ismuch smaller than the path differences induced by the passage of a lightbeam through the atmosphere under normal meteorological conditions andany intereference pattern will therefore be swamped by fluctuationscaused by the atmosphere.

[0012] Although atmospheric turbulence will rapidly wreck the structureof an optical interference pattern the temporal coherence of a lightbeam is fairly well insensitive to atmospheric effects. This is becausethe effective temporal coherence length of photon bunches within a lightpulse is much greater than the path length variations due to densitydifferences. Therefore, there will be an “optical signature” associatedwith the timing of individual groups of photons arriving at a detectorthat represent the RetroReflection from each surface of an opticalsystem.

[0013] In order for the structure of the target optical sight to beresolved the light pulses generated by the light source will have acertain size limit depending on the target optical system. Bearing inmind that light can travel 30 centimetres in one nanosecond then toresolve components that are separated by 15 centimetres (outward andreturn journey of the light beam means this distance is effectively 30cm) the source will have to generate light pulses which are equal to orless than one nanosecond in duration. Light pulses of approximately suchduration or less are hereinafter referred to as “ultrashort” pulses.Note: Since optical components are often much closer then 15centimetres, much shorter pulses are required, of the order of tens offemtoseconds.

[0014] According to the present invention there is provided an apparatusfor detecting and analysing an optical system consisting of a number ofpartially reflecting surfaces comprising

[0015] i) light generating means for generating and emitting a pluralityof ultrashort (as herein defined) source pulses of light towards theoptical system;

[0016] ii) a detector for detection of reflected light signals, thedetector being capable of discriminating the arrival of reflected lightsignals to a time interval of the order of the length of the emittedpulses;

[0017] iii) synchronisation pulse generator means which is arranged toprovide a timing reference against which the arrival time of eachdetected reflected signal can be measured, and;

[0018] iv) signal processing means arranged such that the time intervalbetween the arrival of each reflected signal and the temporally adjacentsynchronisation pulse can be computed

[0019] wherein

[0020] v) the apparatus is operated such that the mean number of photonsper source pulse that are reflected by the optical system and collectedby the detector is ≦1, and

[0021] vi) the signal processing means is arranged to generate ahistogram of reflected pulses as a function of the computed timeinterval and to compute the relative spatial positions of the reflectingsurfaces within the optical system.

[0022] Correspondingly, according to the present invention, there isprovided a method of detecting and analysing an optical systemconsisting of a number of partially reflecting surfaces comprising thesteps of:

[0023] i) generating and emitting a number of ultrashort (as hereindefined) source pulses of light towards an optical system;

[0024] ii) detecting the light signals reflected by the optical system;

[0025] iii) generating a timing reference consisting of a series ofsynchronisation pulses, and;

[0026] iv) computing the time interval between the arrival of eachreflected signal and the temporally adjacent synchronisation pulse

[0027] wherein

[0028] v) step (i) above is operated such that the mean number ofphotons per source pulse that are reflected by the optical system andcollected by the detector is ≦1, and said method comprises the furthersteps of:

[0029] vi) generating a histogram of the reflected signals as a functionof the computed time interval, and;

[0030] vii) computing the relative spatial positions of the reflectingsurface within the optical system.

[0031] The apparatus works by emitting a series of ultrashort lightpulses which strike the target optical system thereby producing a numberof reflected signals whose time of arrival at the detector can becomputed by correlation with a suitable system synchronisation pulse.The synchronisation pulse could be the emitted light pulse itself or anindependent timing pulse within the apparatus' electronics. Over timethe apparatus will build up a histogram of reflected pulses as afunction of the timed interval between each of the detected reflectedsignals and the temporally adjacent synchronisation pulse. Since returnsfrom different reflecting surfaces within the target optical system willeach be separated by different time intervals from their temporallyadjacent synchronisation pulse the spatial separations of the sourcesurfaces can be calculated.

[0032] In order that the detector within the apparatus is not swampedwith too many photons the system needs to be operated in atime-correlated single photon detection mode, i.e. statistically themean number of photons in each source pulse reflected back to thedetector by the target optical system needs to be less than or equal toone. In order to achieve such a low photon detection rate a suitably lowpowered light source is used. In operation this means that a number ofsource pulses may have to interact with the optical system before aphoton is returned to the detector. Conveniently a photon detection rateof one detected photon for every ten source pulses will result in a wellbehaved system.

[0033] Conveniently, the apparatus can be time correlated if the emittedlight pulses are used as the synchronisation pulses such that the lightgenerating means and the detector provide the start/stop pulses for thesignal processing timing circuitry. In this way the relative separationsof reflecting surfaces on the optical system can simply be derived.

[0034] Maximum detector count rate is achieved when the apparatus is setup to be “reverse” time correlated since the timer is triggered onlywhen a photon is detected. Reverse time correlation results in a “timereversal” of the data—shorter time delays are from returns further awayfrom the detector than longer time delays. See FIG. 3. FIGS. 3.1 and 3.2show a simplified version of the apparatus analysing an optical system.The light generating means 100 fires a series of light pulses at thetarget 102 which reflects some of the incident light energy back to thedetector 104. The target is shown to comprise two optical elements 106,108. A timeline 110 represents what the detector “sees”.

[0035] In FIG. 3.1 the apparatus is shown detecting the first surface106. The reflected signal produces a photon counting event 114 withinthe detector 104 which triggers the timing circuitry within the signalprocessing electronics (not shown), the start pulse. The stop pulse forthis timing circuitry is provided by the next emitted light pulse whichrepresents the synchronisation pulse 112. In FIG. 3.1 therefore the timedifference measured by the apparatus is shown on the timeline 110 as theperiod 116.

[0036] In FIG. 3.2 the apparatus is analysing the second optical surface108. The initial photon counting event 114 therefore appears furtherdown the timeline and so the time gap 118 between the start pulse 114and the stop pulse 112 is smaller. In other words optical surfaces thatare further away from the detector result in a shorter time interval.

[0037] If the source pulses are sufficiently short in duration and theinterval between the target surfaces sufficiently great then eachsurface will produce a separate return whose relative separations aresufficient to characterise the target. If the source pulses are longerand/or the surfaces closer together then the returns will overlapproducing a longer envelope pulse. Since the time amplitude shape ofeach source return conforms to that of the source pulse the envelopepulse can be split into its individual components by applying a suitablemathematical de-convolution model to the reflected returns data. Leadingedge fitting processes, wavelet and derivative analysis are all suitablemodels for recovering the structure of the target optical system.

[0038] Conveniently, an ultrashort pulse diode laser, such as aPicoQuant diode laser, can be used to generate the light pulses.

[0039] Many optical systems have components which are separated by aslittle as 5 millimetres and so preferably the light source should alsoideally be capable of generating pulses of around {fraction (1/30)}nanosecond (≈33.3 picoseconds) in duration. It should be noted howeverthat the apparatus will probably function satisfactorily even if thelight source is not capable of resolving individual surfaces withseparations of around 5 mm. This is because the optical componentswithin the optical system will have larger scale structurescorresponding to the separations between groups of components.Furthermore, the use of a de-convolution technique such as describedabove will enable the structure of the target optical system to bededuced.

[0040] In order that the reflections generated by successive pulses fromthe light generating means do not overlap when they reach the detector,the outgoing pulses should preferably have a minimum separation. Forreflecting surfaces of separation z then the separation y betweensuccessive pulse maxima should be y ≧2z. This equates to a pulserepetition rate f ≦c/2z, where c is speed of light. So for the casewhere the optical components are separated by 5 millimetres the pulserepetition rate must be less than 30 GHz.

[0041] Conveniently the signal to noise ratio can be increased, therebyaiding data acquisition rates, by transmitting light pulses whosewavelengths correspond to the Fraunhofer absorption lines in the solarspectrum.

[0042] Suitable detectors for the detection of reflected pulses arediode detectors or photomultiplier tubes. It is important to choosedetectors with a fast impulse response time and as short a recovery time(the period of time which the detector takes to “recover” from detectingan incident light pulse) as possible. The effects of these two factorsare discussed later. Since the detectors will be “off-line” for periodsof time it is preferable that the signal processing means build up ahistogram of reflected pulses as a function of the timed length betweenthe synchronisation pulse and the detected reflected signals in order tobuild up the optical signature.

[0043] Return pulses can conveniently be separated into very short timeintervals of the order of femto- to nanoseconds by using a Time toAmplitude Converter (TAC) which will place return pulses received by thedetector into time “bins” from which the target signature can bederived. The output from the TAC and signal processing electronics couldbe displayed graphically to allow identification to be performed by eyeor more preferably the output can be analysed by a suitable computerprogram (similar to a sonar signature or drug spectrogram identificationprogram) capable of identifying the signature with respect to a storeddatabase of known target signatures.

[0044] Conveniently the speed of detection can be increased by makingthe apparatus cover a much wider field of view by either raster scanningor using an alternative detector such as a charge coupled detectorarray.

[0045] The apparatus could be made more covert if desired by arrangingthe light generating means to be capable of frequency hopping between anumber of different and distinct light frequencies.

[0046] The apparatus described is capable of detection, analysis andidentification. Preferably the ability to range an optical system can beadded to the apparatus either by operating the apparatus in conjunctionwith a separate rangefinder or by transmitting and detecting usingfirst-pulse logic.

[0047] As well as being used to detect and identify remote opticalsystems the apparatus described could be used to check the positioningof optical elements within a precision optical system such as atelescope or microscope, i.e. it could be used for quality control.

[0048] Embodiments of the invention will now be described by way ofexample only with reference to the accompanying drawings in which:

[0049]FIG. 1 shows the invention according to a first configuration(“monostatic” configuration)

[0050]FIG. 2 shows the invention according to a second configuration(“bistatic” configuration)

[0051]FIG. 3 shows diagrammatically the process of time correlatedphoton counting FIGS. 4-7 show various experimental and theoreticalresults from using the apparatus of the invention on various targets,namely:

[0052]FIG. 4 shows a comparison of the results of using the apparatus ofthe invention against three different types of optical systems

[0053]FIG. 5 is a theoretical plot of Counts versus optical path for anideal signature from an optical system comprising eleven separatecomponents with an ultra-fast detector

[0054]FIG. 6 shows the actual signature obtainable from the same set upas in FIG. 6 with current detector technology

[0055]FIG. 7 shows the results of applying a de-convolution technique tothe optical signature

[0056]FIG. 1 shows an example of a “monostatic” configuration for theinvention, i.e. a configuration in which there is a single opticalsystem for both the probe beam output and the return beam collection.This monostatic configuration is the version of the invention tested anddescribed in the later figures.

[0057] In FIG. 1, a semiconductor laser 1 emits 70 picosecond pulses ata repetition rate of 20 MHz and a power level of around 1 mW (The laser1 used was a PicoQuant PDL 800 with a wavelength of 640 nanometres). Theprobe beam 3 produced by the laser 1 passes through a collimating lens 5and then through a polarising beamsplitter 7. The polarised beam thenpasses through a telescope 9 and is collimated by the telescope'sobjective lens 11. Finally the beam passes through a quarter wave plate13 before continuing onto the target (not shown).

[0058] The return signal 15 (which consists of reflected outward pulses)passes once again through the quarter wave plate 13. Thus thepolarisation of the return signal 15 is rotated by 90° with respect tothe outward probe beam 3 which enables the probe beam 3 and returnsignal 15 to be separated by the beamsplitter 7. The beamsplitter 7reflects the return signal 15 into the detector 17.

[0059] The laser 1 is controlled by a laser output controller 19. Thelaser output controller 19 and detector 17 are linked to timingcircuitry 20 which uses the technique of time correlated photon countingin order to resolve the structure of the target. Time correlatedcounting is a well understood technique and here the detector 17 (andtherefore the return signal 15) provides the start pulses for the timingcircuitry and the laser controller 19 provides the stop pulses. Note:this timing technique results in a time reversal in the data such thatshorter time delays are from returns further away in the detector thanthose which give rise to longer time delays (see FIG. 3).

[0060] The apparatus is operated in a single photon detection mode andso in order to analyse the target optical system the return signals mustbe integrated over time and therefore a histogram of reflected signalsversus time delay is formed. Either this can be displayed graphicallyfor visual analysis (not shown) or the data can be analysed by thesignal processing unit 21 in order to identify the “optical signature”of the target system. The skilled man will realise that by reducing therecovery time of the detector (i.e. that time during which the detectoris “recovering” after receiving a signal) will enable the system tooperate at higher photon count and source pulse emission rates so thatthe integration time needed to produce a useful optical signature willgo down.

[0061]FIG. 2 shows a bistatic configuration for the invention in whichtwo separate optical systems are used, a transmitting telescope 23 and areceiving telescope 25. This configuration is not markedly differentfrom the monostatic configuration (In FIG. 2 like numerals are used toidentify identical elements of the device to those shown in FIG. 1) butthe use of two separate optical systems removes the requirement for apolarising beamsplitter and a quarter wave plate increasing throughputand potentially the signal to noise ratio.

[0062] In both the monostatic and bistatic configurations a narrow bandfilter 27, tuned to pass the light wavelength of the laser 1, can beadded to the system to reduce the amount of noise received.

[0063] It will be obvious to the skilled man that there are equivalentconfigurations to the ones shown in FIGS. 1 and 2 using alternativecomponents, such as reflecting optics, holographic filters, gratingfilters, fibre filters etc. Furthermore, in FIG. 1 the quarter waveplate 13 could actually be placed anywhere on the telescope side of thebeamsplitter 7. Also, even though a cube beamsplitter is shown there areother functionally equivalent configurations.

[0064]FIG. 4 shows experimental traces of three different opticalsighting systems at a range of 1.2 kilometres. The signatures aredisplayed for the case when the first surfaces of the targets are atexactly the same distance from the apparatus in order to demonstratethree things. Firstly the signatures are all different which impliesthat they could be used to positively identify their respective optics.Secondly, the distribution of the families of peaks reveals internalstructure of the optics (note that the peaks do not appear in the samepositions for all three systems) and thirdly that the distance betweenthe first and last peaks of each sight gives an optical path length ofthe target.

[0065]FIGS. 5 and 6 show how theoretical results match up to currentlyachievable results. FIG. 5 shows the signature expected from a complexoptical system comprising eleven separate optical components with adetector having a standard deviation of 0.01 nanoseconds. The effectsdue to all eleven optical surfaces present within the sight are added toproduce the envelope which resembles a series of spikes. FIG. 6 on theother hand shows the trace produced when using the best detectorcurrently available which has a standard deviation of only 0.16nanoseconds. However, even on this basis useful information can beobtained. This is because the system will be detecting the coarseroptical signatures due to different groups of optical surfaces within anoptical system. Therefore, as long as the operator or signal processingelectronics are aware of the detection capabilities of the system thenuseful comparisons against other systems resolved at the same level canbe achieved.

[0066] It will be obvious therefore to the skilled man that thedetecting capability of the invention will improve as detectors with afaster impulse response are developed.

[0067] As discussed above de-convolution techniques could be applied tothe detected optical signature in order to resolve the signal into itsindividual components. FIG. 7 shows how an envelope signal cansuccessfully be de-convolved using in this case a leading edge fittingprocess to reveal the four individual optical components (Surfaces 1-4)within the optical system.

1) An apparatus for detecting and analysing an optical system consistingof a number of partially reflecting surfaces comprising i) lightgenerating means for generating and emitting a plurality of ultrashort(as herein defined) source pulses of light towards the optical system;ii) a detector for detection of reflected light signals, the detectorbeing capable of discriminating the arrival of reflected light signalsto a time interval of the order of the length of the emitted pulses;iii) synchronisation pulse generator means which is arranged to providea timing reference against which the arrival time of each detectedreflected signal can be measured, and; iv) signal processing meansarranged such that the time interval between the arrival of eachreflected signal and the temporally adjacent synchronisation pulse canbe computed wherein v) the apparatus is operated such that the meannumber of photons per source pulse that are reflected by the opticalsystem and collected by the detector is ≦1, and vi) the signalprocessing means is arranged to generate a histogram of reflected pulsesas a function of the computed time interval and to compute the relativespatial positions of the reflecting surfaces within the optical system.2) An apparatus as claimed in claim 1 wherein the emitted light pulsesare used as the synchronisation pulses such that the emitted anddetected pulses/signals provide the start/stop pulses for the signalprocessing circuitry. 3) An apparatus as claimed in claim 2 wherein theapparatus is reverse time-correlated 4) An apparatus as claimed in anypreceding claim wherein the signal processing means additionally appliesa de-convolution algorithm to the detected reflected light signals. 5)An apparatus as claimed in any of claims 1 to 4 wherein the lightgenerating means is a pulse laser diode. 6) An apparatus as claimed inany preceding claim wherein the wavelength of the light pulsescorresponds to a Fraunhoffer wavelength 7) An apparatus as claimed inany preceding claim wherein the detector is a Photomultiplier tube 8) Anapparatus as claimed in any of claims 1-7 wherein the detector is adiode detector. 9) An apparatus as claimed in any preceding claimwherein the signal processing means includes a Time to Amplitudeconverter 10) An apparatus as claimed in any of the claims 1-9 whereinthe output from the signal processing means is analysed by a computerprogram capable of identifying the optical signature of the targetoptical system with respect to a stored database of known opticalsignatures 11) An apparatus as claimed in any preceding claim whereinthe apparatus is capable of raster scanning a field of view 12) Anapparatus as claimed in any of the claims 1-10 wherein the detectorcomprises a Charge Coupled Detector array 13) An apparatus as claimed inany of the preceding claims wherein the light generating means iscapable of frequency hopping. 14) An apparatus as claimed in anypreceding claim wherein the apparatus is operated in conjunction with arangefinder. 15) A method of detecting and analysing an optical systemconsisting of a number of partially reflecting surfaces comprising thesteps of: i) generating and emitting a number of ultrashort (as hereindefined) source pulses of light towards an optical system; ii) detectingthe light signals reflected by the optical system; iii) generating atiming reference consisting of a series of synchronisation pulses, and;iv) computing the time interval between the arrival of each reflectedsignal and the temporally adjacent synchronisation pulse wherein v) step(i) above is operated such that the mean number of photons per sourcepulse that are reflected by the optical system and collected by thedetector is ≦1, and said method comprises the further steps of: vi)generating a histogram of the reflected signals as a function of thecomputed time interval, and; vii) computing the relative spatialpositions of the reflecting surface within the optical system.